1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, or a facsimile, which employs an image forming method such as an electrophotography, an electrostatic recording technique, an ionography, or a magnetic recording technique.
2. Description of the Related Art
In recent years, there has been a growing demand for image forming apparatuses that are versatile enough to be able to form images on a wide range of recording media with different thicknesses, such as thin sheets, thick sheets, postcards, and envelopes. Many image forming apparatuses adopt a so-called intermediate transfer technique because it is advantageous in meeting the above demand. In general, according to the intermediate transfer technique, a toner image formed on a latent-image carrier, such as a photosensitive member, is subjected to a primary transfer onto an intermediate transfer body, which is an image carrier, and then the toner image on the intermediate transfer body is subjected to a secondary transfer onto a recording medium by passing the recording medium between the intermediate transfer body and a secondary transfer member that makes contact with the intermediate transfer body to form a transfer nip. In such an image forming apparatus, when the recording medium enters a secondary transfer portion (contact portion between the intermediate transfer body and the secondary transfer member), the speed of the intermediate transfer body, which is driven at a constant speed, fluctuates, and an image is distorted at the first transfer portion. Consequently, the image that is eventually formed on the recording medium degrades. This problem is noticeable particularly if the recording medium is relatively thick.
In order to overcome this problem, Japanese Patent Application Laid-open No. H4-242276 discloses a technique for changing a space between the intermediate transfer body and the secondary transfer member according to the thickness of the recording medium to suppress speed fluctuations of the intermediate transfer body that occur when the recording medium enters the contact portion between the intermediate transfer body and the secondary transfer member or when the recording medium exits the contact portion. By doing so, it is possible to suppress, to some degree, the degradation in the image that is eventually formed on the recording medium.
However, in the image forming apparatus described in Japanese Patent Application Laid-open No. H4-242276, the thickness of the recording medium is measured before the recording medium enters the secondary transfer portion, and the space between the intermediate transfer body and the secondary transfer member can be changed based on the measurement result (i.e., according to the thickness of the recording medium). Here, consider the timing of moving the secondary transfer member when images are to be formed on a plurality of recording media. If the recording media to be conveyed to the secondary transfer portion have the same thickness (i.e., recording media with different thicknesses are not mixed), then the secondary transfer member is moved by a predetermined distance based on the information about the measured thickness of a recording medium when a printing job is started, images are formed on the recording media during the job while maintaining the space between the intermediate transfer body and the secondary transfer member, and the secondary transfer member is returned to its original position when the job ends. In other words, if the recording media to be conveyed to the secondary transfer portion have the same thickness (i.e., recording media with different thicknesses are not mixed), because the secondary transfer member does not move while the toner image is being subjected to primary transfer onto the intermediate transfer body, speed fluctuations of the intermediate transfer body due to the movement of the secondary transfer member do not occur.
However, if recording media with different thicknesses are mixed, it becomes necessary to move the secondary transfer member while the toner image is primary-transferred onto the intermediate transfer body. Consequently, the intermediate transfer body suffers from speed fluctuations as a result of moving the secondary transfer member to change the space between the intermediate transfer body and the secondary transfer member. Thus, the images that are eventually formed on the recording media are distorted as in the conventional technology. The speed fluctuations produced in the intermediate transfer body as a result of moving the secondary transfer member to change the space between the intermediate transfer body and the secondary transfer member are likely to be induced mainly from vibrations generated in the intermediate transfer body by a shock occurring when the movement of the secondary transfer member is started or stopped or from a change in pressure due to a change in the space between the intermediate transfer body and the secondary transfer member. Therefore, if the recording medium is, for example, a thick sheet, the secondary transfer member is moved a longer distance, and the shock or the pressure changes greatly when the movement of the secondary transfer member is started or stopped. Consequently, it leads to larger speed fluctuations in the intermediate transfer body. In short, if a plurality of recording media to be conveyed to the secondary transfer portion includes, for example, a thick sheet, the images suffer from marked degradation.
The above-mentioned problem of the image quality degradation due to the movement of the secondary transfer member occurs not only when the secondary transfer member is moved towards the surface of the intermediate transfer body while the toner image is being transferred at the primary transfer portion. For example, the same problem may occur when a moving member, such as a cleaning member or a transfer member, is moved towards the surface of the image carrier while the image on the surface of the image carrier is being transferred onto the intermediate transfer body (or onto the recording medium in the case of the direct transfer method). Furthermore, the same problem may occur when the moving member is moved towards the surface of the image carrier, for example, while a latent image is being written on the surface of the image carrier.
In short, there is a risk that the above-mentioned problem occurs when a moving member is moved towards the surface of the image carrier while an image is being formed on the image carrier or while the image on the image carrier is being transferred to another transfer medium (e.g., the intermediate transfer body or a recording medium).